Effects of Magnetic Field, Temperature and Impurity on Crystal Growth of Calcite
|關鍵字:||晶體成長;方解石;磁場效應;溫度效應;雜質效應;鍶離子;亞鐵離子;Crystal growth;Calcite;Magnetic field effect;Temperature effect;Impurity effect;Strontium ion;Iron(II) ion||公開日期:||2009||摘要:||在工業程序上，冷卻水的管路內及鍋爐壁上的結垢是很常見的問題，一旦結垢的問題的產生，除了造成熱交換的效率降低，也會導致堵塞管路。由於水中溫度及pH值的變化，使得在壁上容易結垢，其中又以碳酸鈣為結垢的主要成分，而方解石不僅僅是碳酸鈣多晶型的其中一種，更是常見的結垢成分。而通常結垢問題都發生在高溫的設備中，像是熱交換器、鍋爐，因為碳酸鈣隨著溫度提高溶解度越低，所以高溫下的水常常是形成過飽和溶液的條件，導致結垢於壁面上。在眾多的防垢技術中，其中一種是利用磁場來防垢，雖然防垢效果眾說紛紜，但市面上以磁場來抑制管內結垢的方法已有80年的歷史。而在程序用水中含有不同種類的雜質離子，可能會加速或抑制方解石的成長。本研究採用定組成法，於攪拌結晶槽中進行方解石成長實驗，進而討論磁場施加與否、溫度的高低與雜質的不同對方解石成長速率的影響。 方解石晶體成長實驗結果發現，在未受磁的情況下，常溫(25 °C)下的過飽和溶液，方解石晶體的成長速率可隨過飽和度與pH值增加而提升。無磁場作用下，常溫(25 °C)下的過飽和溶液中含有不同的濃度的鍶離子/亞鐵離子，改變雜質濃度從0.5至2.0 ppm，每當濃度增加2倍時，鍶離子造成方解石成長速率上升約10 %；而亞鐵離子導致方解石成長速率下降約22 %。在未受磁情況下，溶液溫度從25 °C提升40 °C將使得方解石晶體的成長下降約60 %。亦在未受磁情況下，高溫(35 °C)下受雜質影響之趨勢與常溫下相仿，方解石成長速率隨著鍶離子濃度的上升而增快；而亞鐵離子濃度對成長速率為抑制之作用。 在磁場存在之條件下，溶液中過飽和度、pH值、溫度對方解石成長速率亦有影響。在過飽和度為0.8至1.2、pH值為8.5到9.5、溫度為25 °C到40 °C，磁場雖都可完全抑制方解石晶體成長，但在低過飽和度、低pH值與高溫的情況下，方解石被磁場完全抑制所需時間較短。 受磁20小時的常溫(25 °C)溶液中含有微量鍶離子/亞鐵離子濃度時，改變鍶離子濃度從0.5至2.0 ppm，成長速率隨著鍶離子濃度的增加而提升；當亞鐵離子濃度從0.5增至2.0 ppm時，而成長速率隨著亞鐵離子濃度的增加而降低。 本實驗亦探討受磁20小時的高溫(35 °C)溶液中含有微量鍶離子/亞鐵離子濃度時，磁場、高溫與雜質三者之交互作用。在磁場作用下，當溶液中有雜質存在時，方解石成長速率不會被完全抑制，將鍶離子濃度從0.5提高至2.0 ppm，成長速率隨著濃度的提升而增大；將亞鐵離子濃度提高從0.5增至2.0 ppm時，成長速率隨著濃度的提升而下降。方解石晶體在磁場與雜質的環境中，溫度的提升，可使成長速率下降。
The buildup of scale deposit inside cooling water pipes and boiler walls is a common and costly problem for many industrial processes. The scale deposit leads to a reduction in heat transfer efficiency and to a partial or complete blockage of water flow. Due to the variation in pH and temperature, the scale is formed, which composes of calcium carbonate in the majority. Calcite is one of the polymorphs of calcium carbonate and the most common form of scales. The scale formation usually occurs in the equipments operated at higher temperature, such as heat exchanger and boiler. Since calcium carbonate becomes less soluble with increasing temperature, the hot water turns into supersaturated conditions with regard to CaCO3, which deposits on the hot surfaces. There are several scale prevention methods, among which the magnetic treatment is the most controversial one. However, the magnetic treatment device has been available in the market for about 80 years. Moreover, the process water contains various kinds of impurity ions, which would accelerate or inhibit the growth of calcite crystals. In this research, a constant – composition method was adopted to measure the calcite growth rate in a stirred crystallizer. The principal subject of this research is to investigate the effects of magnetic field, temperature and impurity on the calcite growth.n the series of calcite growth experiments at 25°C, the calcite growth rate increased with increasing supersaturation and pH value without magnetic treatment. In the absence of magnetic field, the enhancement of calcite growth rate was about 10 % when the concentration of strontium ion was varied by a factor of 2, i.e., from 0.5 to 1.0 ppm or from 1.0 to 2.0 ppm. On the other hand, the reducing percentage of growth rate was about 22 % when the concentration of iron(II) ion was altered by a factor of 2. As to the temperature effect, the calcite growth rate decreased with an increase in temperature, from 25 to 40°C. When the supersaturated solution at 35°C contained various concentrations of strontium or iron(II) ion, we found that an increase in Sr2+ concentration was accompanied by an increase in calcite growth rate. However, the effects of Fe2+ concentration on the calcite growth rate were opposite to that of Sr2+.n the presence of magnetic field, the calcite growth rate was also influenced by the solution variables, including supersaturation, pH value and temperature. Although the growth rate was suppressed completely in the presence of magnetic field for the supersaturation from 0.8 to 1.2, the pH value from 8.5 to 9.5, and the temperature from 25 to 40°C, the time needed to suppress the growth rate completely is shorter under high temperature, low pH and supersaturation.hen the supersaturated solution containing a small quantity of strontium or iron(II) ion at 25°C was magnetized by the Descal-A-Matic DC-1 for 20 hours, the calcite growth rate would not be suppressed completely. The growth rate increased with increasing Sr2+ concentration, from 0.5 to 2.0 ppm. On the other hand, the growth rate decreased with increasing Fe2+ concentration.he experiment was further conducted to investigate the interaction of magnetic field, temperature and impurity on the growth of calcite crystal. The supersaturated solution containing impurity was magnetized for 20 hours at 35°C before growth experiment. Under these circumstances, the growth rate of calcite would not be suppressed completely either. When the concentration of Sr2+ increased from 0.5 to 2.0 ppm, the growth rate also increased. Furthermore, the trend of Fe2+ concentration on growth rate was opposite. As to the temperature effect, the higher temperature led to a reduction in the calcite growth rate, when magnetic field and impurities were present.
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